The present invention relates to measuring instruments, and more particularly to apparatus for measuring deformation.
The apparatus according to the invention can find application as a reference standard during metrological certification of resistance strain-gauge transducers or as an instrument for measuring deformation in the course of elasticity, strength and other tests.
The advancements in resistance strain-gauge transducers call for the provision of means for their accurate metrological verification. The development of design methods in the theory of elasticity and plasticity requires, in turn, the provision of an apparatus for accurate measuring the field of deformation on the surface of a test specimen for experimental checking corresponding mathematical models.
Known methods and apparatus for measuring deformation are based on the registration of relative displacement of markers, such as linear scatterers, arranged on the surface of a test specimen, or on taking record of a change in the period of a diffraction grating formed on the surface of the test specimen. A disadvantage of such known apparatus resides in the fact that they employ recorders that fail to provide required sensitivity and accuracy of measurements.
There is known an apparatus for measuring strain (U.S. Pat. No. 3,628,866, Cl. 356-32) comprising a laser cooperating with an optical system for forming two beams of light, a marker in the form of a portion of a diffraction grating applied to the surface of a test specimen, and a recorder or a display unit including a lens for receiving diffracted light and a screen positioned in the focal plane of the lens.
During deformation measurement of one of the two light beams is directed onto the diffraction grating. The light beams scattered by the diffraction grating are received by the lens to be focused on the screen. As a result of strain induced in the test specimen, the period of the diffraction grating changes causing a change in the angle between the light beams scattered by the diffraction grating. This in turn causes relative linear displacement of the light pattern focused on the screen. The value of this relative displacement of spot lights on the screen helps determine the amount of deformation to which the test specimen has been subjected.
A disadvantage inherent in the above apparatus is that it features relatively low sensitivity and accuracy due to limited resolution power and aberrations of the lens.
Also known is an apparatus for determining changes in spacing between two positions of interest (cf. U.S. Pat. No. 4,050,818, IPC G 01 b 11/16) comprising a laser, two markers in the form of linear scatterers formed at preselected points on the surface of a test specimen, a recorder of interference pattern resulting from interference of the light scattered by the markers, the recorder comprising a photodetector and a strip chart recorder. The apparatus may be used for measuring deformation of a surface between the two markers, the amount of deformation being determined through relative displacement of the markers.
The apparatus operates in the following manner. The markers are illuminated by a light beam produced by the laser. The light scattered by the markers tends to interfere. The relative displacement of the markers causes the movement of interference pattern relative to the photodetector, the latter producing an electric signal fed to the strip chart recorder. The value of the relative displacement of the markers can thereby be determined using the known initial distance therebetween, as well as by the laser beam wavelength and the number of interference fringes that have passed through the photodetector.
However, the above apparatus has low sensitivity and accuracy, since the method of measuring employed in conjunction with the apparatus fails to provide accuracy in excess of 0.1 of one interference fringe. In addition, outside vibrations or incidental displacement of the apparatus relative to the test specimen result in unwanted movement of the interference fringes and consequently to measurement errors.
Further known are apparatuses for measuring deformation (cf. Patents of U.K. Nos. 1364607 and 1454340, IPC G 10 B 11/16) comprising a laser and an optical system for forming light beams, the laser and the optical system being arranged along a common optical axis, markers in the form of a diffraction grating impinge on the surface of a test specimen, and a recorder of an interference pattern resulting from the light scattered by the diffraction grating.
The foregong apparatuses operate in the following manner: The optical system forms two intersecting light beams produced by the laser. A transparent replica grid (according to U.K. Pat. No. 1454340) is positioned at the plane of intersection of the light beams, the replica being obtained by a contact method from the diffraction grating deformed together with the surface of the test specimen; alternatively, a photographic plate (according to U.K. Pat. No. 1364607) bearing the visual recording of the deformed periodic pattern may be used. The recorder, in this case a photo camera, records an interference pattern resulting from interference of two diffracted orders belonging to different light beams illuminating the replica grid (or the photographic plate). The amount of deformation is determined by decoding the interference pattern recorded on the photographic plate.
However, the known apparatuses are disadvantageous in that they feature low resolution capacity and precision. In the first place, casting a replica from the diffraction grating deformed on the surface of the test specimen is a very time-consuming process (U.K. Pat. No. 1454340), the results of measurements being further affected by such destabilizing factors as changes in the temperature of the surrounding medium. Secondly, the apparatus for measuring deformation according to U.K. Pat. No. 1364607 makes use of a lens for recording the interference pattern produced by the deformed diffraction grating, the lens affecting the accuracy of measurements due to low resolution power optical aberrations. And thirdly, an accuracy of not higher than 0.1 of one interference fringe is attained during decording the interference patterns recorded on the photographic plates.
It is an object of the present invention to increase the accuracy and sensitivity of an apparatus for measuring deformation.
Another object of the invention is to speed up the process of measuring deformation.
Still another object of the invention is to provide for automation in the course of deformation measurements.